Actinomycetes.

The term Actinomycetes is used to indicate organisms belonging to the Actinomycetales, a major subdivison of the Prokaryotae, the kingdom which comprises all organisms with a prokaryotic cell. In contrast to the organisms of the other four kingdoms (Protista, Fungi, Plantae and Animalia) which all have eukaryotic cells.

The classification of the Prokaryotae is a complex issue. Major subdivisions are the bacteria (Schizophyta), the blue green algae (Cyanophyta) and the Actinomycetales. The latter are sometimes called the "higher bacteria", organisms possessing properties intermediate between the fungi and the bacteria. The Actinomycetes are gram-positive organisms that tend to grow slowly as branching filaments, resembling fungi, as their filamentous growth forms mycelial colonies. For this reason they were long regarded as fungi, as is reflected in their name: aktino(gr)= ray, mykes(gr)= mushroom (=fungus).

The Actinomycetes are divided into several families

• The family Actinomyceteae comprises two genera: Actinomyces and Nocardia. A few species are pathogenic.  A.Israelii causes actinomycosis in man, A.Bovis in cattle.
• The family Mycobacteriaceae has a single genus, Mycobacterium, which contains several pathogenic species causing diseases such as leprosy and tuberculosis.
• The family Streptomyceteae comprises several organisms found in the soil. They are rarely pathogenic. In contrast, several species of the genus Streptomyces produce antibiotics. Erythromycin is produced by Streptomyces erythreus, streptomycin by S.Griseus, aureomycin by S.Aureofaciens, oleandomycin by S.Antibioticus, spiramycin by S.Ambofaciens.

Sometimes the name "Actinomycetes" is used restrictively for members of the genus Actinomyces only, instead of all members of the Actinomycetales.